Saturn's icy moon Enceladus is thought to host a liquid ocean beneath its frozen surface that could be hospitable to life. Credit: NASA

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Saturn’s frozen moon Enceladus is a tantalizing world—many scientists are increasingly convinced it may be the best place in our solar system to search for life. NASA’s Cassini spacecraft, currently orbiting Saturn, has made intriguing observations of icy jets spewing from a suspected underground liquid ocean on the mysterious world that might be hospitable to alien life.

Cassini’s tour is due to wind down in 2017, and scientists badly want to send a dedicated mission to Enceladus to look for signs of life. In fact, some have already started seriously thinking about exactly how they might do this—including planetary scientist Carolyn Porco, who is the imaging team leader for Cassini. Earlier this month, she gathered a group of researchers including oceanographers, organic chemists and astrobiologists at the University of California, Berkeley, to strategize how to search for extraterrestrials on Enceladus—which, according to Porco, “is a total bitch of a problem to solve.”

Although Enceladus is small in size and shrouded in a thick shell of ice, it appears to be a habitable world: It has a source of energy from friction created by its orbit around Saturn, organic compounds that are building blocks for life and a liquid water ocean underneath all that ice. But just because Enceladus may be hospitable to life does not mean life exists there; it will take much more work to definitively prove it. At the Berkeley meeting, scientists laid out the data Cassini has collected for Enceladus—they discussed analyses of its geysers, measurements of its ice shell, ideas on what its ocean chemistry might be like, and more. Yet even with all the newest data and models scientists have, they are not even close to detecting organisms on Enceladus—hence the need for a space mission.

Finding life there would be a profound revelation that we are not alone in the cosmos. Furthermore, the discovery of organisms—or the lack thereof—could answer the subtler mystery of how life started on Earth. Researchers at the meeting presented two major opposing theories about how life here originated (in the ocean versus on land), and the group discussed how exploring Enceladus would inform this debate. “It would be a test of one of the ideas about the origin of life,” Porco says—specifically, the proposition that Earth’s species sprang in the sea. For example, if organisms exist in Enceladus’s ocean and presumably arose there, it would support the theory that life began on Earth in hydrothermal vents (hot, nutrient-rich, deep-sea vents on the ocean floor) rather than in patches of water on land.

Enceladus could also teach us about genesis in our solar system in other critical ways. “You’re not just searching for life, you’re searching for an understanding of the nature of that life, and how it compares to life on Earth,” says Chris McKay, a planetary scientist at NASA Ames Research Center. For instance, if we discover that creatures on Enceladus are nothing like those on Earth—if their biochemistry is completely different—then it would likely mean that the two forms of life arose separately and independently, and thus, that aliens might be likely to exist other places as well. “If life started at least twice in our solar system, then you know the universe is full of life,” McKay says. Or, if we find out that Enceladus organisms and Earth organisms are made in identical ways, it may indicate that life originated someplace else, and was carried to both worlds. If Enceladus is barren, however, it could support the theory that life needs an environment on dry land to get started, not an ocean. Regardless of what a mission to Enceladus might discover, the answer will tell us something fascinating.

SKIMMING GEYSERS

Enceladus has over 90 geysers that spew plumes of salty water vapor, organic compounds and ice particles from the underground ocean into the air. These present a great opportunity for a visiting spacecraft, which would not have to land to search for life (which is much more difficult and expensive) but could simply fly through the geysers to capture samples. “The plume is coming right out of the ocean,” McKay explains, “So why would we want to land? We can get the freshest stuff, coming right from the source.”

Yet even if life exists on Enceladus, it may or may not show up in plume samples. If the pelagic ocean on Earth (that is, the open water away from the shore or seafloor) is an analogue for Saturn’s icy ocean moon, then the outlook is depressing—the pelagic zone has an extremely low density of life even on our planet. “If we had this in Enceladus’s ocean, it would be very hard to even pick up an organism,” Porco says. Scientists would need to sample a ridiculously large amount of water in order to capture any organisms.

Thankfully, a few months ago a microbiologist told Porco about decades-old scientific research that makes her optimistic about finding life in the plumes. At the Berkeley meeting, she described this research on a process called “bubble scrubbing” that occurs in Earth’s oceans—and it could make quite a difference in Enceladus’s geysers. It turns out that wherever bubbles rise through water, they scrub the water column so that organisms and organic materials become concentrated at the surface. And when the bubbles burst (like in ocean spray or in Enceladus’s jets), they eject those microbes in the spray. So if life exists on Enceladus, its plumes may contain a much greater concentration of organisms than the rest of its ocean—all thanks to bubbles. “Even if the ocean on Enceladus starts out being as microbially poor as the pelagic ocean on Earth, which is the worst case, we still have a chance of seeing lots of organisms in the plumes,” Porco says. Still, this scenario immediately presents another issue: A spacecraft must find a way to capture a sample without smashing the delicate organisms to bits as it makes a high-speed pass through the jets.

SEEKING SIGNS OF LIFE

Once a spacecraft collects a sample from Enceladus, how will scientists test it for life? The process is more complex than simply searching for something that is alive—after all, researchers have argued over the definition of life for years. In the case of hunting for extraterrestrials scientists must get creative. “If you went to Mars and found a dead rabbit on the ground, it’s not alive, but it is compelling evidence of life,” McKay says. “So we’re not searching for something that’s alive but searching for the molecules that life uses. In other words, we’re looking for the body of the dead rabbit.”

The molecules that McKay and other scientists consider most important are amino acids—the building blocks for proteins. “They occur on comets and meteorites, so if there’s a primordial soup on Enceladus, it should have amino acids,” McKay says. “They’re so incredibly useful and so good in water that life would be pretty dumb not to use them.” At one point in the Berkeley meeting, however, Porco brought up a critical point: What if Enceladus organisms aren’t made of amino acids? McKay replied, jokingly, “Then we’re sunk and nature is perverse. We should all just give up and become poets.” What he meant is there’s a large consensus in the scientific community that amino acids will be useful in the hunt for life—and if such thinking turns out to be wrong, well “then we’re even dumber than we thought we were,” McKay says.

Another important signature scientists want to detect is lipids, which cells use to build their outside walls. “It’s a similar story to amino acids,” explains Alfonso Davila, a research scientist at the SETI Institute and Ames. “They’re something you’d expect to be present at the origin of life and you’d expect cells to use them.” Scientists will need to do more than simply detect amino acids and proteins on Enceladus—both of those molecules exist on their own in many environments, with or without life. But astrobiologists can target distinct structures and distributions of amino acids and lipids they think are unique to life. “We’re looking for molecules and structures that life makes, that are distinctly different from the random mess that chemistry makes,” McKay notes.

Some other possibilities on the search list are large organic compounds as well as photos of actual organisms taken by the spacecraft of samples from the plume. That might be, for example, images of an organism swimming or eating. Such a find might be the most direct evidence of something “alive” but many researchers have doubts about the plausibility of imaging organisms, Davila says. “It’s one of those high-risk, high-reward experiments. The likelihood of a negative result is very high,” he adds. “It’s very hard to tell the difference between a cell and a dot that is just a particle.” This issue was hotly debated at the meeting but some scientists sounded more hopeful—they discussed new techniques they are exploring to reliably image Enceladus’s microbes (if they exist).

Ultimately, scientists think it will probably take a combination of evidence to show that they have actually found life. And, of course, cost and technology will constrain the experiments they are able to perform. The hunt will be incredibly complicated, especially considering that organisms on Enceladus may not look or operate anything like they do on Earth. “We’re walking a thin line between what we know based on Earth life and what we expect life would be like otherwise,” Davila says. “It’s one of the things that prevents us from coming up with a good strategy.”

Or, to reiterate Porco’s observation—“It’s a total bitch of a problem to solve.”

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